The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
With methods and apparatus of the initially named kind, an object detection signal is usually generated when the transmitted light beam or the reflected light beam is interrupted by an object intruding into the protected zone so that, for example, the energy (light intensity, brightness) detected by the receiver is reduced by a predetermined threshold value with respect to the energy detected with a non-interrupted beam path. That region of the receiver is designated as the detection region on which the reflector image comes to lie when no object is present in the beam path. On the evaluation of the detected reflector image, the shape, position or specific encodings of the reflector image can, for example, also be taken into account. Objects of all types, inter alia also persons, are to be understood as an object in the sense of the present application.
It is necessary for the reliable recognition of an object to recognize with high accuracy the interruption of the light beam transmitted or reflected. It is problematic in this connection that an object entering into the visual range of the receiver can reflect or remit both the light transmitted by the transmitter and the light reflected by the reflector, in particular also scattered light or other interfering light, such that the light reflected or remitted at the object is incident onto the detection region of the receiver. In this case, a light intensity can be detected by the receiver in the detection region despite an interruption of the beam path which corresponds to the light intensity with an uninterrupted beam path so that an object detection signal is erroneously not generated.
To avoid these problems, with known processes, for example, polarization filters are used for the optical reduction of the interfering light signals. Since, depending on the application, however, the objects interrupting the transmitted light beam can also comprise depolarizing or polarization changing materials, a reliable recognition of corresponding objects is also not always ensured by this measure.
The problems described also in particular occur when the objects have reflecting regions. In particular when the reflecting regions are present at inclined surfaces or at radii such as rounded edges present at the surface of the object, the probability that a light beam reflected at the object is incident on the receiver is relatively large. The light intensity generated by the reflected regions on the receiver in this case is frequently of the same order of magnitude as the light intensity of the reflector image generated by the reflector with an uninterrupted beam path so that an uninterrupted beam path is erroneously assumed in these cases in the energy evaluation of the reflector image.
Other interfering light signals independent of the transmitted light can also be directly or indirectly incident on the detection region of the receiver, for example via an object, and can thus simulate an uninterrupted beam path despite the interrupted beam path.